xs_send(3) send a message part on a socket


int xs_send (void *socket, void *buf, size_t len, int flags);


The xs_send() function shall queue a message created from the buffer referenced by the buf and len arguments. The flags argument is a combination of the flags defined below:


Specifies that the operation should be performed in non-blocking mode. If the message cannot be queued on the socket, the xs_send() function shall fail with errno set to EAGAIN.


Specifies that the message being sent is a multi-part message, and that further message parts are to follow. Refer to the section regarding multi-part messages below for a detailed description.


A successful invocation of xs_send() does not indicate that the message has been transmitted to the network, only that it has been queued on the socket and Crossroads have assumed responsibility for the message.

Multi-part messages

A Crossroads message is composed of 1 or more message parts. Crossroads ensures atomic delivery of messages; peers shall receive either all message parts of a message or none at all. The total number of message parts is unlimited except by available memory.

An application that sends multipart messages must use the XS_SNDMORE flag when sending each message part except the final one.


The xs_send() function shall return number of bytes in the message if successful. Otherwise it shall return -1 and set errno to one of the values defined below.



Non-blocking mode was requested and the message cannot be sent at the moment.


The xs_send() operation is not supported by this socket type.


The xs_send() operation cannot be performed on this socket at the moment due to the socket not being in the appropriate state. This error may occur with socket types that switch between several states, such as XS_REP. See the messaging patterns section of xs_socket(3) for more information.


The context associated with the specified socket was terminated.


The provided socket was invalid.


The operation was interrupted by delivery of a signal before the message was sent.


Sending a multi-part message.

/* Send a multi-part message consisting of three parts to socket */
rc = xs_send (socket, "ABC", 3, XS_SNDMORE);
assert (rc == 3);
rc = xs_send (socket, "DEFGH", 5, XS_SNDMORE);
assert (rc == 5);
/* Final part; no more parts to follow */
rc = xs_send (socket, "JK", 2, 0);
assert (rc == 2);


This man page was written by Martin Sustrik <m[blue][email protected]m[][1]>, Martin Lucina <m[blue][email protected]m[][2]> and Pieter Hintjens <m[blue][email protected]m[][3]>.